Modern, high performance flat panel displays are mostly based on Liquid Crystal (LC) technology and are often referred to as Liquid Crystal Displays (LCDs). Flat Panel Displays (FPDs) and LCDs use glass as both a substrate and a cover sheet with a thin LC layer encapsulated in-between the two sheets of glass. The glass sheets used in the manufacturing of FPDs are quite large as indicated in the table below (glass dimensions are in mm):
Gen 5Gen 5.5Gen 6Gen 71000 × 12001300 × 15001500 × 18001800 × 20001200 × 13001500 × 18501850 × 21001600 × 19001870 × 22001900 × 2200
In particular, TV and computer FPD screens contain a large number of picture elements, i.e. pixels, with the typical pixel size for a computer screen FPD being 80×240 μm. Pixels are formed by a Thin Film Transistor (TFT) pattern, which is deposited on the substrate in multiple photo-lithography steps. Defects as small as 15×15 μm in the glass substrate, in particular pits, disrupt the TFT deposition process resulting in defective pixels or a defective TFT array. These glass defects, in the substrate or cover glass, may adversely affect the transmission of light through the finished FPD resulting in an unacceptable FPD product and adversely effect the TFT patterning process resulting in shorts, open circuit or electrically defective thin film transistors.
Some examples of glass defects include a pit which is a small indentation in the glass; an inclusion or embedded foreign particle, such as platinum, stainless steel, silica or a gas bubble; an adhesion chip, such as a glass chip fused with the glass surface and not removable by washing; a scratch; and edge chip; or a distortion, such as a localized refractive index non-uniformity or a localized error of flatness/thickness which introduces an undesirable lens like effect to the substrate. These defects vary in shape, and may range in size from ˜15×15 μm to a few hundred microns.
The discovery of defects in a final inspection of FPD panels is troublesome, due to the high material and labour costs of manufacturing a defective FPD. Therefore, it would be beneficial for the glass manufacturer to inspect the glass prior to shipping it to FPD fabrication plants.
Known methods of inspecting large, flat, non-patterned media typically fall into two main categories: (a) imaging systems using an imaging element, such as a charge coupled device or CCD, with pixels of a smaller size than required by the inspection resolution (object plane resolution) and an imaging lens to provide optical magnification to match the camera pixel size to a desired object plane resolution or (b) laser scanners using a laser beam focused down to the spot size corresponding to the desired object plane resolution and a single detector.
Prior art related to the category imaging methods includes U.S. Pat. No. 6,633,377 entitled Dark View Inspection System for Transparent Media; U.S. Pat. No. 6,437,357 entitled Glass Inspection System including Bright Field and Dark Field Illumination; U.S. Pat. No. 6,208,412 entitled Method and apparatus for determining optical quality; U.S. Pat. No. 5,642,198 entitled Method of Inspecting Moving Material; and U.S. Pat. No. 5,493,123 entitled Surface Defect Inspection System and Method. Typically, for web inspection, line scan CCD cameras are used with the camera pixel size ranging from 7 μm to 13 μm. Cameras of 7 μm pixel size and 8 kilo-pixels (8192) resolution are commercially available. In order to achieve a desired defect detection accuracy of 15×15 μm, the object plane resolution of the imaging system should be at least 20×20 μm yielding a lens magnification of 20 μm/7 μm=2.85. If the object plane size is 2,000 mm, the total number of pixels in the object plane is 2,000 mm/20 μm=100,000 and thus the required number of cameras is 100,000/8 kpixels=13. When taking into consideration the expense of thirteen 8 k CCD cameras, the total cost of the inspection systems based on line scan cameras is very high.
Moreover it is difficult and costly to provide CCD camera system with a lens that does not limit the camera pixel resolution, in particular for a large CCD sensor size of 0.007 mm×8,192=57.4 mm. If an ideal diffraction limited lens is used with the 8 k camera, the required F-number would be 3.3, which is on the boundary of practicality to design the lens with an image plane size of 57.4 mm, F-number of 3.3 and optical point spread function (PSF) of 7 μm across the entire field of view—even for monochromatic light application. In practice the lens PSF limits the imaging system performance by limiting the resultant optical resolution. Conversely if one attempts to apply a 13 μm pixel size camera, the required F-number for an ideal lens would be 5, which is less demanding. Due to silicon die size limitations, these types of cameras are typically only available with a 2 k (2024) pixel resolution. In this case, to cover the object plane of 2,000 mm one would require 50 cameras, making an inspection system prohibitively expensive. Therefore, when using small CCD pixel size, optics limit the resultant imaging system resolution and when using large CCD pixel size, the result is a prohibitively large number of cameras.
Prior art related to the laser scanning methods includes U.S. Pat. No. 5,452,079 entitled Method of and Apparatus for Detecting Defect of Transparent Sheet as Sheet Glass. The limiting performance-cost product of a typical CCD based optical imaging system can be overcome by utilizing an optical scanner.
One disadvantage of using optical scanners for LCD glass inspection is a scanning speed limitation imposed by the scanner mechanics. Another disadvantage is that, in order to maintain a web speed of 100 mm/s, multiple scanners are required. Furthermore, a single optical scanner is unable to cover a glass width of 2000 mm. Therefore, multiple scanners are required, which increases the cost of the inspection system.
It is, therefore, desirable to provide a novel method and apparatus for inspecting flat media.